The invention relates to an optical system, in particular a projection exposure system for microlithography, in particular having a slot-shaped image field or non-rotational-symmetric illumination,
a) having an optical element comprising at least one chamber that is sealed from atmospheric pressure and is enclosed by boundary surfaces and that has a fluid filling, wherein at least one of the boundary surfaces is exposed at least partially by illumination light;
b) fluid source that has a fluid connection to the chamber via a fluid supply line; and
c) control device for the pressure of the filling liquid.
Such a system is described in U.S. Pat. No. 4,676,631 A. In that case, to adjust the projection enlargement of a projection exposure system, an optical element is used that is disposed between a projection optics system and a laser. The optical element comprises two transparent plates that form the boundary of a gastight cavity with a common mounting. The transparent plates are deformed by means of a controllable internal gas pressure in a defined manner so that they act like a lens of controllable positive refractive power. Only the adjustment of the projection enlargement, that is to say of a rotational-symmetric imaging property, is possible with such an arrangement. Non-rotational-symmetric imaging properties, for example, to compensate for non-rotational-symmetric imaging errors, are, on the other hand, not adjustable.
It is precisely in projection exposure systems, but also in other optical systems, that it has been found that their imaging quality is often reduced by a variable astigmatism. Variable astigmatism arises, for example, as a result of non-rotational-symmetric light-induced effects, such as heating or xe2x80x9ccompactionxe2x80x9d, that result in a corresponding astigmatic expansion or refractive-index distribution in the optical element. Variable astigmatic lens effects, dependent for example on the pump intensity, also occur in solid-state laser media. In that case, either a birefringence is induced by the pump light or the oscillating laser light, or a birefringence already present in the laser medium is amplified, which often results in a variable astigmatism in the laser resonator.
It is known, for example, from EP 0 678 768 A2 to compensate for the light-induced astigmatic imaging error in a lens by an astigmatism artificially produced by means of a non-rotational-symmetric annealing of the lens material. However, due to the poor thermal conduction of most lens materials, this compensation mechanism has undesirably long time constants.
In addition, for certain optical applications, further circumstances are conceivable in which it is desirable to produce a variable astigmatism in a defined manner.
It is therefore the object of the present invention to develop an optical system of the type mentioned at the outset in such a way that it is possible to produce in said system an additional variable astigmatism that can be used, for example, for compensation purposes.
This object is achieved, according to the invention, by a system, wherein the enclosed chamber is configured in such a way that a change in the fluid pressure inside the at least one chamber results in a change in non-rotational-symmetric imaging properties of the optical element that have an n-fold symmetry relative to the optical axis of the optical element, where n is greater than 1.
The invention therefore continues the basic idea, disclosed in U.S. Pat. No. 4,671,631 A, of changing the optical properties of an element by means of the gas pressure and, as a result of the pressure-induced change in the non-rotational-symmetric imaging properties with n-fold symmetry, now also opens up the possibility of compensating corresponding imaging errors that occur.
The change in the fluid pressure inside the at least one chamber may result in a change in the astigmatic properties of the optical element. Astigmatism is a particularly frequently occurring non-rotational-symmetric imaging error having two-fold symmetry that can be corrected by such an optical element.
In a preferred refinement of the invention, at least that region of the surfaces forming the boundary of the chamber that is irradiated by illumination light is at least partially formed by an elastically deformable medium, the edge contour of the elastically deformable region being non-rotational-symmetric. An elastically deformable medium having a non-rotational-symmetric edge contour bulges in selected, mutually perpendicular planes in such a way that different curvatures are present at those points. Depending on the pressure of the fluid filling, the focal length ratio of such an astigmatic optical element changes in mutually perpendicular meridional planes. In total, this results in an easy-to-implement compensation element for a variable astigmatism.
The edge contour has an n-fold symmetry relative to the optical axis of the optical element, where n is greater than 1. The edge contour that can be adapted in this way to the symmetry of the imaging error to be corrected results in the possibility of correcting in a controlled manner certain n-fold imaging errors, but leaving other imaging properties having a different n-fold symmetry unaltered.
The edge contour may be elliptically shaped. Such an edge contour can easily be specified and therefore manufactured in an automated way. The ratio of the imaging properties in the mutually perpendicular meridional planes can be adjusted in a simple manner by means of the length ratio of the major axes of the ellipse.
Alternatively, the edge contour may have the shape of a polygon. Such edge contours can easily be produced.
Preferably, the elastically deformable optical medium is held in its edge region by a holding device, the shape of the holding surface with which the optical medium is in contact with the holding device imposes the edge contour of the elastically deformable surface region. In this way, a non-rotational-symmetric edge contour of an elastically deformable medium can easily be achieved.
The optical medium may be a pellicle. Such optical membranes have a very good optical quality and can be bent by relatively moderate fluid pressure changes.
Alternatively, the optical medium may be a quartz plate or a CaF2 plate. Quartz plates have good optical properties and can be produced with such small thicknesses that they can be deformed in a defined manner even by moderate pressure changes. CaF2 plates have good transparency, in particular, in the ultraviolet wavelength range of interest for projection exposure.
The optical medium may have a reflecting coating. The optical element can then be used as an imaging-correcting mirror.
An alternative embodiment is one wherein at least one region of a surface of the surfaces forming the boundary of the chamber is irradiated by illumination light and is formed by at least one rigid optical surface having different curvature in mutually perpendicular planes. In this embodiment, use is made of the effect that the change in the deflection of transmitted light rays as a consequence of a pressure-induced refractive index change in the fluid filling depends on the absolute value of the angle of incidence or angle of emission of the optical interfaces of the chamber for the fluid filling. In this embodiment, therefore, changing the fluid pressure also makes it possible to adjust the astigmatism of the optical element.
At the same time, the optical element is preferably formed from a combination of at least two optical components that each comprise at least one chamber that is sealed from atmospheric pressure and is enclosed by boundary surfaces, that has a fluid filling and that is irradiated by illumination light, the optical components having, at least in the region of one surface of the surfaces forming the boundary of the respective chambers in each case at least one optical surface having different curvature in mutually perpendicular planes; and an independent control of the pressure of the fluid filling in the chambers assigned to the optical components is ensured by means of the control device. Larger regions of astigmatisms to be adjusted can be covered by using a plurality of optical components having fluid-pressure-dependent astigmatism.
The optical element can be designed in this case so that, given equal pressure in the fluid filling in the chambers assigned to the optical components, it has essentially rotational-symmetric imaging properties. The optical element then has essentially no astigmatism in a neutral state in which the same pressure prevails in both chambers. In an optical arrangement in which rotational-symmetric imaging properties are required, astigmatisms of light-induced origin, for example, can then be compensated for on the basis of this neutral state.
Alternatively, the optical element can be designed so that, given equal pressure in the fluid filling in the chambers assigned to the optical components, it has astigmatic imaging properties. Such an optical element can be used in situations in which astigmatic imaging properties, for example for imaging strip-type objects, are required or in which the starting point is an astigmatism that has to be compensated for from the outset around which a fine adjustment is then possible.
The optical surface having different curvature in mutually perpendicular planes may be a surface of a cylindrical lens. As a result of such a configuration, decoupling of the adjustable imaging properties is possible in two mutually perpendicular planes. Under these circumstances, the respective gas pressure is adjusted in one of the two chambers to adjust the imaging properties in one of said planes. The imaging properties in the plane perpendicular thereto remain unaffected thereby.
The cylindrical lens may be a plano-convex cylindrical lens. The use of convex surfaces forming the boundary of the chamber offers advantages in the fluid supply since, in the region of a casing enclosing the optical components, that is to say in the edge region of the optical components, convex optical components are in each case at the greatest distance from one another. This is therefore advantageous, in particular, since the fluid-filled chamber should be designed as small as possible in order, on the one hand, to achieve a required pressure change or deformation by a small supply or removal of fluid and, on the other hand, to keep the optical element compact.
The control device may have a signal connection to a sensor arrangement that monitors the imaging properties of the optical element and/or the optical system, the control device impressing a pressure in the fluid filling as a function of the transmitted signal data of the sensor arrangement. Such a monitoring of the imaging properties results in the possibility of regulating the fluid pressure or the fluid pressures for the purpose of automatic compensation for imaging errors.
The sensor arrangement may have a position-sensitive sensor. Monitoring the imaging properties of the optical element and/or of the optical system is possible in a simple manner with such a sensor. An inexpensive embodiment of such a position-sensitive sensor is, for example, a quadrant detector.
Preferably, the position-sensitive sensor is a CCD array. A precise determination of the imaging properties and the use of known image processing algorithms for their monitoring is possible with such a sensor.
The control device may be designed so that it is capable of producing both underpressures and overpressures. This increases the adjustment range of the astigmatic imaging properties of the optical element. If an optical element having a boundary surface that can be elastically deformed by means of the fluid pressure is used, a concavely shaped boundary surface relative to the enclosed chamber can be achieved by an underpressure and a convexly shaped boundary surface can be achieved by an overpressure.
Preferably, the fluid is a gas. A fine adjustment of the shape change can be produced by means of a change in pressure if an elastically deformable boundary surface is used.
If an optical element irradiated by illumination light and a noble gas is used as fluid, particularly advantageous optical properties are produced, in particular with respect to the transmission of the fluid. Any reaction of the filling gas with the optical components or the casing surfaces is virtually ruled out.
Alternatively, the fluid may be a liquid. A liquid enables a relatively rapid adjustment of the imaging properties of the optical element if an elastically deformable boundary surface is used since only a small supply or removal of fluid volume compared with a gas is necessary to deform the boundary surface.